1. Field of the Invention.
The present invention relates to a washing machine and more particularly to a rotatable plastic wash basket having sufficient rigidity to prevent undue vibration and deflection.
2. Description of the Prior Art.
Rotatable wash baskets for use in automatic washers may be subject to various types of forces during basket spin and may have particular dynamic responses to forced vibrations which may occur during basket spin. The types of forces experienced by the basket typically include a centrifugal force created by an evenly distributed load of clothes, a localized imbalance force created by an unevenly distributed load of clothes and a reactive force applied at the top lip of the basket by spinning water in a balance ring. The wash basket is typically supported by a hub member which is interconnected with a drive shaft for supporting and rotating the wash basket. The localized imbalance force resulting from an unevenly distributed load of clothes creates a moment at the hub member, and if uncorrected, may create undesired vibration in the wash basket during rotation. Preferably, the reactive force resulting from the spinning water in the balance ring will create a substantially equal and opposite moment at the hub member for correction of the localized imbalance such that undesired vibration is prevented. An eccentric spin condition must exist for the balance ring to function therefore some small difference typically exists between the moment created by the reactive force at the hub member and and the moment created by the localized imbalance force at the hub member. However, due to physical size limitations in the balance ring, in the case of large localized imbalance forces large differences in moments may exist. These differences result in an overall imbalanced condition for the wash basket which subjects the rotating wash basket to forced vibrations during spin. These forced vibrations will have a frequency equal to the spin frequency (basket revolutions/sec) of the rotatable wash basket.
Typically, rotatable wash baskets for use in an automatic washer are constructed of metal. For example, U.S. Pat. No. 4,890,465 shows a metallic rotatable wash basket having a single wall basket structure including a single wall base structure. U.S. Pat. No. 2,921,460 shows a metallic rotatable wash basket having a single wall basket structure including a single wall base structure having an additional rigid metallic bottom plate attached to the base for providing base rigidity. Because of the inherent stiffness and strength of metal, rotatable wash baskets made of metal are sufficient to withstand the various forces to which a rotatable wash basket may be subjected.
It is also well known to utilize plastic rotatable wash baskets in an automatic washer. The use of a plastic material for a rotatable wash basket may offer various advantages over metal including lower cost and corrosion resistance. However, the strength of plastic is such that simple single wall wash basket construction, similar to metallic wash basket designs, may not have adequate strength and stiffness. Plastic processing limitations prevent wash basket designers from simply increasing the thickness of the plastic walls to improve wash basket strength. Therefore, to provide adequate strength and stiffness in a plastic basket, various wash basket reinforcement configurations are employed. These wash basket reinforcement configurations are typically intended to provide additional strength to the bottom or base of the wash basket. In particular, the prior art shows the use of reinforcing ribs located on the base of a plastic rotatable wash basket or the use of a rigid metallic bottom plate attached to the base of a plastic rotatable wash basket.
For example, U.S. Pat. No. 5,012,658 illustrates the use of reinforcing ribs located on the base of a rotatable plastic wash basket for providing the necessary strength and stiffness. The use of reinforcing ribs, however, may have several disadvantages. The use of external ribs on the base of a wash basket may cause the formation of an undue amount of suds between the rotatable basket and an imperforate, non-rotatable tub. This suds condition may cause the automatic washer to fail to adequately perform various functions. Furthermore, ribbed base designs may still not offer equivalent strength and stiffness as compared to metallic wash baskets having metallic base structures and rotatable plastic basket having a ribbed base may be inadequate to withstand the various forces to which a rotating wash basket is subjected.
Additionally, U.S. Pat. No. 4,483,161 and U.S. Pat. No. 4,444,027 illustrate the use of a metal reinforcing plate secured to the base of a plastic rotating wash basket for providing the required stiffness and strength. The use of a metallic reinforcing plate may also have several disadvantages. The use of a reinforcing plate requires the use of fasteners which increases the difficulty of assembly. In addition, a metallic reinforcing plate may create corrosion problems. Finally, the addition of a reinforcing plate and fasteners may increase the cost of the rotatable wash basket.
In both plastic and metallic rotatable wash baskets, the deflection and the dynamic response of the rotatable wash basket may be influenced by several factors including the overall stiffness of the wash basket. In particular, the basket should preferably be designed such that the lowest natural frequency of the wash basket structure is greater than the spin frequency of the rotatable wash basket so that undue vibration may be avoided. Given the inherent stiffness and strength of metal, undue deflection or vibration has typically not been an issue in the design of metallic baskets. Typically, the lowest natural frequency of a metallic basket is higher than the spin frequency of the basket and therefore excitation of the basket at its natural frequency is avoided. However, since plastic has less inherent strength and stiffness than metal, plastic baskets typically require reinforced structure to prevent undue deflection and vibration.
Engineering analysis performed by the inventors has shown that the base which forms the bottom wall of the basket is critical in dictating the stiffness of the entire basket. Therefore, a plastic rotatable wash basket having a base geometry which provides adequate stiffness to the entire basket may provide a basket design having its lowest natural frequency higher than the spin frequency of the basket such that undue vibrations are not experienced during basket spin. Prior art, however, appears to teach the use of relatively thick walls in the vertical cylindrical portion of the plastic wash basket for achieving stiffness and strength. This manner of providing basket stiffness for preventing undue deflection and vibration is relatively ineffective and expensive. Therefore, a basket having a base geometry which provides adequate stiffness may have a cylindrical outerwall portion of the basket having a wall thickness less than a wash basket having a base with inadequate stiffness such that a basket having an adequately stiff base may use less material than a basket having an inadequately stiff base.
There is, therefore, a need for a plastic rotatable wash basket for use in an automatic washer having a structure sufficient to withstand the forces applied to the basket, but which does not create a sudsing problem or require additional reinforcing plates. Additionally, there is a need for a plastic rotatable wash basket having a base sufficiently stiff such that the lowest natural frequency of the rotatable basket is greater than the spin frequency of the rotatable wash basket. Furthermore, there is a need for a plastic basket construction which minimizes the basket wall thickness thereby being of relatively low cost.